Oracle 9i Database Administration in 10 Minutes

- Asim Abbasi (Author)

Chapter 4: Understanding Oracle Database

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How Oracle Instance Uses Redo Log Files?

Every database should have two or more redo log groups. Each redo log group should have at least two or more redo log files called members.

Figure 15: Redo log files in action.
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Within one Redo log group, all redo log files are mirror copies of each other and its recommend that they all should reside on different physical hard drives to increase the fault tolerance factor. Having mirror copies of the original Redo Log file within a group is called Multiplexing of Redo Log files. Whatever data changes are happening in the database, all gets written to the Redo Log files. In other words you can say it’s the “Undo” copy of the original database. As shown in Figure 15, Oracle Instance (or more precisely, the Log Writer process) will start writing these changes in data from Redo Log group 1, once it gets filled up, Oracle instance will move to group 2 and finally group 3. Once group 3 finishes up, Oracle instance will start all over gain from group 1. That’s the reason we say that Redo Log files get utilized in a circular fashion.

To avoid overwriting of Redo Log files, you should run the Oracle Instance in Archive Log Mode. Whenever Oracle is running in this mode, redo log files gets archived well before they get overwritten.

Chapter 4: Understanding Oracle Database

Page #: 1 | 2 | 3 | 4 | 5 | 6 | 7

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Oracle 9i Database Administration in 10 Minutes